The Parked Domains feature allows you to point a domain to your main site / public_html folder.

You can still use the redirect option under cPanel -> Parked domains and redirect a parked domain name to a sub-folder for your main domain. However, this will not keep the parked domain name in the URL. You will be redirected to a subfolder for the main domain.

In order to have a domain name pointed to a specific subfolder on your web hosting account, you  need a hosting account that allows using addon domain functionality. This feature is available in higher hosting plans like Hosting PLUS, Semi Dedicated hosting, etc. If you are using the Standard shared hosting by SiteGround, but want to use the addon functionality you need to upgrade your hosting plan from the Order section of your Customer area.  Please click here to find more information about the Addon Domains.